Cationic alkylzirconium complexes based on a tridentate diamide ligand: New alkene polymerization catalysts

Article abstract of DOI:10.1021/om960073k

Alkyl abstraction from {Me₃SiN(CH₂CH₂-NSiMe₃) ₂}ZrR₂(N₃ZrR₂; R = CH₂Ph, Me) using B(C₆F₅)₃ affords cationic alkyl complexes stabilized by a diamide ligand. The ionic η²-benzyl adduct decomposes slowly to give a cationic cyclometalation product, which coordinates the [PhCH₂B(C₆F₅)₃]^- anion; the methyl cation coordinates the anion [MeB(C₆F₅)₃]^- via a Zr?Me-B interaction. The complexes exhibit moderate ethene polymerization activity.

Full text of DOI:10.1021/om960073k

2672
Organometallics 1996, 15, 2672-2674
Ca tion ic Alk ylzir con iu m Com p lexes Ba sed on a
Tr id en ta te Dia m id e Liga n d : New Alk en e P olym er iza tion
Ca ta lysts
Andrew D. Horton,* J an de With, Arjan J . van der Linden, and
Henk van de Weg
Shell Research and Technology Centre, Amsterdam, Postbus 38000,
1030 BN Amsterdam, The Netherlands
Received February 5, 1996X
Summary: Alkyl abstraction from {Me3SiN(CH2CH2-
NSiMe3)2}ZrR2 (N3ZrR2; R ) CH2Ph, Me) using B(C6F5)3
affords cationic alkyl complexes stabilized by a diamide
ligand. The ionic η2-benzyl adduct decomposes slowly
to give a cationic cyclometalation product, which coor-
dinates the [PhCH2B(C6F5)3]- anion; the methyl cation
coordinates the anion [MeB(C6F5)3]- via a Zr‚‚‚Me-B
interaction. The complexes exhibit moderate ethene
polymerization activity.
precursors9-14 and the expected increased electrophi-
licity of diamide cations compared to analogues with
polydentate nitrogen ligands. A program to probe the
potential of diamide ligands in polymerization has now
lead to the first alkene polymerization catalysts based
on cationic diamide complexes. Recent reports13,14 of
neutral group 4 adducts of the new tridentate ligand
[Me3SiN(CH2CH2NSiMe3)2]2- lead us to report here our
work involving cationic group 4 alkyl adducts of this
ligand.
The search for alternatives to group 4 metallocenes
as electrophilic “uniform site” catalysts for alkene
polymerization is currently of great interest.1,2 Much
attention has focused on nitrogen-containing ligands
([L]- or [L2]2-), such as porphyrins,3 tetraaza[14]-
annulenes,4 tetradentate Schiff base ligands,5 (hydroxy-
phenyl)oxazolines,6 and benzamidinates.7 Unfortu-
nately ethene polymerization activities (for L2MCl2/
methylaluminoxane or [L2MR]+; M ) Ti, Zr, Hf) have
been generally disappointing.4-7 In contrast, and de-
spite the application of dicyclopentadienyl and cyclo-
pentadienylamide catalysts in kiloton-scale industrial
processes, diamide complexes (the next family in this
series) have received little attention in the patent8 or
scientific literature.9-14 This is surprising considering
the facile synthesis of diamide complexes as catalyst
New crystalline dialkyl precursors of putative cationic
complexes have been prepared from the readily avail-
able dichlorozirconium complex 115,16 using standard
alkylation methodology (Scheme 1).17,18 The observation
of 1H NMR resonances (C6D5Br) for inequivalent benzyl
(2, -20 °C) or methyl groups (3, 25 °C), as well as four
different backbone dimethylene hydrogens,19 is consis-
tent with coordination of the amino nitrogen to zirco-
nium in a trigonal bipyramidal structure. Fluxional
exchange of the environments of the axial and equato-
(11) For recent work on complexes of other d0 metals with chelating
diamide or triamide ligands, see the following and references therein:
(a) Vaughan, W. M.; Abboud, K. A.; Boncella, J . M. J . Am. Chem. Soc.
1995, 117, 11015. (b) Freundlich, J . S.; Schrock, R. R.; Cummins, C.
C.; Davis, W. M. J . Am. Chem. Soc. 1994, 116, 6476.
(12) Andersen, R. A. Inorg. Chem. 1979, 18, 2928.
(13) Cloke, F. G. N.; Hitchcock, P. B.; Love, J . B. J . Chem. Soc.,
Dalton Trans. 1995, 25.
(14) Clark, H. C. S.; Cloke, F. G. N.; Hitchcock, P. B.; Love, J . B.;
Wainwright, A. P. J . Organomet. Chem. 1995, 503, 333.
X Abstract published in Advance ACS Abstracts, May 1, 1996.
(1) (a) Sinclair, K. B.; Wilson, R. B. Chem. Ind. (London) 1994, 857.
(b) Brintzinger, H. H.; Fischer, D.; Mu¨lhaupt, R.; Rieger, B.; Waymouth,
R. M. Angew. Chem., Int. Ed. Engl. 1995, 34, 1143.
(2) (a) Stevens, J . C.; Timmers, F. J .; Schmidt, G. F.; Nickias, P.
N.; Rosen, R. K.; Knight, G. W.; Lai, S.-Y. European Patent Application
416 815 (Dow, 1990). (b) Cannich, J . A. M.; European Patent Applica-
tion 420 436 (Exxon, 1990). (c) Devore, D. D.; Timmers, F. J .; Hasha,
D. L.; Rosen, R. K.; Marks, T. J .; Deck, P. A.; Stern, C. L. Organome-
tallics 1995, 14, 3132.
(3) Brand, H.; Capriotti, J . A.; Arnold, J . Organometallics 1994, 13,
4469.
(4) Uhrhammer, R.; Black, D. G.; Gardner, T. G.; Olsen, J . D.;
J ordan, R. F. J . Am. Chem. Soc. 1993, 115, 8493.
(5) Tjaden, E. B.; Swenson, D. C.; J ordan, R. F. Organometallics
1995, 14, 371.
(6) Cozzi, P. G.; Gallo, E.; Floriani, C.; Chiesi-Villa, A.; Rizzoli, C.
Organometallics 1995, 14, 4994.
(7) (a) Go¨mez, R.; Green, M. L. H.; Haggitt, J . L. J . Chem. Soc.,
Chem. Commun. 1994, 2607. (b) Herskovics-Korine, D.; Eisen, M. S.
J . Organomet. Chem. 1995, 503, 307. (c) Walther, D.; Fischer, R.; Go¨rls,
H.; Koch, J .; Schweder, B. J . Organomet. Chem. 1996, 508, 13.
(8) (a) Isotactic polypropene formation using {(Me3Si)2N}2MCl2/MAO
has been claimed: Canich, A. M.; Turner, H. W. Patent Application
WO 92/12162 (Exxon, 1992). (b) Sasaki, T.; Shiraishi, H.; J ohoji, H.;
Katayama, H. European Patent Application 571 945 (Sumitomo, 1993).
(9) A zirconium diamide/methylaluminoxane polymerization catalyst
has very recently been reported: Cloke, F. G. N.; Geldbach, T. J .;
Hitchcock, P. B.; Love, J . B. J . Organomet. Chem. 1996, 506, 343.
(10) For very recent examples of group 4 complexes with chelating
diamide ligands, see refs 9, 13, and 14 and the following: (a) Aoyagi,
K.; Gantzel, P. K.; Kalai, K.; Tilley, T. D. Organometallics 1996, 15,
923. (b) Warren, T. H.; Schrock, R. R.; Davis, W. M. Organometallics
1996, 15, 562. (c) Scollard, J . D.; McConville, D. H.; Vittal, J . J .
Organometallics 1995, 14, 5478.
(15) Preparation of 1 proceeds via reaction of Me3SiN(CH2CH2-
NLiSiMe3)2 (N3Li2) with ZrCl4 in Et2O (44% yield). The triamine, N3H2,
is very easily prepared (>95% purity) by reaction of a small excess of
Me3SiCl with HN(CH2CH2NH2)2 in hexane, followed by removal of the
amine hydrochloride byproduct by filtration and reduction to dryness.13
(16) Horton, A. D.; de With, J .; van der Linden, A. J .; van de Weg,
H. Unpublished results.
(17) Synthesis of 2: Toluene (20 mL) at -78 °C was added to a
mixture of 1 (3.1 mmol) and Mg(CH2Ph)2(dioxane)0.5 (3.9 mmol). After
the solution was warmed to 25 °C, the solvent was removed and 2
crystallized from pentane at -40 °C (0.84 g, 45%). Synthesis of 3: To
a suspension of 1 (0.41 mmol) in Et2O (10 mL) at -78 °C was added
an Et2O solution of LiMe (0.83 mmol). After the solution was warmed
to 25 °C, the solvent was removed and 3 crystallized from hexane at
-40 °C (0.104 g, 58%). Synthesis of 5: Toluene (0.5 mL) and
bromobenzene (0.1 mL) were added to a mixture of 2 (0.19 mmol) and
B(C6F5)3 (0.19 mmol) at -30 °C. After the solution was stirred at 25
°C for 16 h, hexane (5 mL) addition afforded a precipitate, which was
washed with hexane and dried, giving pure 5. Typical NMR experi-
ment: C6D5Br (0.7 mL) at -30 °C was added to a mixture of 2 or 3
and B(C6F5)3 (each 0.01 mmol) in a 10 mL bottle at -30 °C, followed
by rapid warming to 0 °C and transfer to an NMR tube. Although 4,
6, and 9 are formed quantitatively (>95% NMR purity), attempts to
isolate the complexes in analytically pure crystalline form have not
been successful.
(18) Reaction of 1 with LiMe in Et2O was recently reported to give
an intractable mixture.13
(19) The complexes were characterized by 1H and 13C NMR spec-
troscopy and (for 2) elemental analysis. The NMR data supports η1-
benzyl coordination in 2: ipso C, δ 146.8 ppm; ZrCH2, δ 72.1, 64.0
ppm (1J CH ) 120, 124 Hz); ortho H, δ 6.86, 6.78 ppm.
S0276-7333(96)00073-8 CCC: $12.00 © 1996 American Chemical Society

Communications
Organometallics, Vol. 15, No. 12, 1996 2673
Sch em e 1
equiv of toluene, to give a single organometallic prod-
uct,17 which may be precipitated with hexane. 1H, 13C,
and 19F NMR spectroscopy20 and elemental analysis
have confirmed that orange solid 5 is the product of
C-H activation of one of the amido SiMe3 groups in 4
(Scheme 2). Ligand activation is reflected in the
observation of four different SiMe resonances (1:1:3:3)
2
and a diastereotopic SiCH2Zr group (δ 0.70, 0.37, J HH
) 13.2 Hz). The zirconium cation is stabilized by
coordination of the benzylborate anion,22 as shown, in
particular, by the downfield ipso carbon resonance (δ
161.0 ppm; free anion, δ 148.6 ppm) and the large value
of ∆δ(m,p-F) of 3.9 ppm (free anion, 2.7 ppm) in the 19F
NMR spectrum.24
In contrast to ionic 4, the methyl complex 6, obtained
from the reaction of dimethyl 3 with B(C6F5)3 in C6D5-
Br,17 exhibits a covalent bonding interaction between
the cation, [N3ZrMe]+, and the [MeB(C6F5)3]- anion
(Scheme 3).25 Anion cordination is reflected in the
downfield location of the BMe resonance (δ 1.38 ppm;
free anion δ 1.13 ppm) and the large value of ∆δ(m,p-
F) of 4.4 ppm.24 The ZrMe group resonates at δ 0.65
ppm (1H NMR) and at δ 55.1 ppm (13C NMR), slightly
upfield of dimethyl 3.
Analogous methyl cations may be obtained by proto-
nolysis (7) or reaction with the trityl reagent (8) in C6D5-
Br solution (Scheme 3).17,20 Cation 7 is stabilized by
NMe2Ph coordination (downfield ortho and para hydro-
gen resonances).17,26 For complex 8, it is not possible
to distinguish between possible weak solvent and anion
coordination to the zirconium cation. The location of
the ZrMe resonance in the complexes (7, δ 0.77 ppm; 8,
δ 0.75 ppm), slightly downfield from 6, may reflect
increased metal electrophilicity (compared to the THF
adduct of 6: δ 0.48 ppm).
Formation of methyl cations 7 and 8 proceeds via an
intermediate species, 9, cleanly obtained using 0.5 equiv
of [Ph3C][B(C6F5)4] (Scheme 3).17,20 Further reaction of
9 with another 0.5 equiv of the reagent affords 8 (20
min, 25 °C). Complex 9 exhibits two ZrMe resonances
(δ 0.51, 2 Me; δ 0.48 ppm, 1 Me), as well as resonances
for two equivalent and symmetric {N3Zr} fragments (2:1
ratio of SiMe3 groups, four backbone methylene hydro-
gens). These data are consistent with a structure in
which a methyl group bridges two [N3ZrMe]+ fragments,
similar to recently reported metallocene complexes,
[{Cp′2ZrMe}2(µ-Me)]+.27 Again, it is clear that a methyl
group of a group 4 dimethyl complex can act as an
effective donor for an electrophilic metal center.
Preliminary reactivity studies of the cationic com-
plexes have revealed moderate ethene polymerization
activity but very low propene activity. A 20-fold excess
of ethene is rapidly polymerized (<5 min, 25 °C) by
C6D5Br solutions of 4 or 6-8 in an NMR tube; the
conversion of 20 equiv of propene (to propene oligomers)
is incomplete after 15 min. In all cases only a small
rial benzyls and of the dimethylene hydrogens in 2,
observed at 25 °C, presumably involves dissociation of
the amino nitrogen, inversion at nitrogen, and recoor-
dination.14 The greater lability of the amino nitrogen
in 2 than in dimethyl 3 (two sharp ZrMe 1H NMR
resonances at 25 °C: δ 0.55 and 0.46 ppm) might reflect
stabilization of the 4-coordinate species by η2-benzyl
coordination.19
Abstraction of a benzyl group from 2 with B(C6F5)3
in C6D5Br or C2D2Cl4 solution cleanly affords a soluble
product, 4;17 in toluene 4 is deposited as a yellow
1
precipitate. The complex has been shown by H, 11B,
13C, and 19F NMR spectroscopy20 to consist of a ben-
zylzirconium cation21 and a noncoordinated [PhCH2B-
(C6F5)3]- anion (Scheme 2).22 The pseudotetrahedral
complex is stabilized by η2-coordination of the benzyl
ligand (giving a maximum 14-electron count at Zr), as
1
shown by the large J CH value of 142 Hz for the ZrCH2
group (δ 68.7 ppm) and the upfield ipso carbon (δ 137.5
ppm) and ortho hydrogen resonances (δ 6.38 ppm).23 An
identical cation (in addition to free NMe2Ph or Ph3CCH2-
Ph) is obtained using [PhMe2NH][B(C6F5)4] or [Ph3C]-
[B(C6F5)4].
Solutions of 4 (with [PhCH2B(C6F5)3]- as anion)
decompose slowly (16 h, 25 °C), with elimination of 1
(20) Selected NMR data (see also text) (1H NMR, C6D5Br, 25 °C,
unless otherwise stated) are as follows. 4: δ 7.24 (p-BzZr), 7.19 (m-
BzZr), 3.36 (BCH2), 3.17, 2.99, 2.81 (4H, 2H, 2H, NCH2), 2.69 (ZrCH2),
0.22, -0.21 (18H, 9H, SiMe3). 13C NMR (C2D2Cl4, -35 °C): δ 136.1,
130.5 (2C, o,m-BzZr), 129.3 (p-BzZr), 52.7, 49.6 (2C, NCH2), 32 (1C,
BCH2), 0.7, -1.2 (6C, 3C, SiMe3). 19F NMR: δ -131.63 (d), -165.13
(t), -167.85 (m). 5: δ 7.2 (o,m-Bz), 6.68 (p-Bz), 3.6-2.2 (NCH2, BCH2),
0.16, 0.12, -0.02, -0.03 (3H, 9H, 3H, 9H, SiMe3). 13C NMR: δ 161.0
(1C, ipso-Bz), 50.9, 50.1, 50.0, 49.9 (1C, NCH2), 34.5 (ZrCH2), 0.9 (1C,
SiMe), -0.6, -0.7, -1.1 (3C, 3C, 1C, SiMe). 19F NMR: δ -131.96 (d),
-162.09 (t), -165.98 (m). 6: δ 3.65 (2H, NCH2), 3.1-2.8 (4H, NCH2),
2.47 (2H, CH2), 0.00, -0.01 (18H, 9H, SiMe3). 13C NMR (C6D5Br, -25
°C): δ 15.7 (BMe). 19F NMR: δ -133.68 (d), -161.26 (t), -165.64 (m).
7: δ 7.24 (m-Ph), 7.12 (p-Ph), 6.88 (o-Ph), 2.69 (s, 6H, NMe2). 13C NMR
(C2D2Cl4, -25 °C): δ 61.5 (ZrMe). 9: δ 3.55, 3.16, 2.84, 2.48 (4H, NCH2),
0.15, -0.07 (36H, 18H, SiMe3).
(24) The value of ∆(m,p-F) (19F NMR) is a good probe of coordination
of [RB(C6F5)3]- (R ) Me, CH2Ph) to cationic d0 metals (values 3-6
ppm indicate coordination; <3 ppm indicates noncoordination): Horton,
A. D. Unpublished results.
(25) Yang, X.; Stern, C. L.; Marks, T. J . J . Am. Chem. Soc. 1994,
116, 10015.
(21) Schematic representation of one of two possible trigonal bipy-
ramidal isomers (axial/equatorial groups exchanged) for 4 and 6-9 is
arbitrary. A single species is observed spectroscopically.
(22) Pellecchia, C.; Immirzi, A.; Grassi, A.; Zambelli, A. Organome-
tallics 1993, 12, 4473.
(23) (a) Latesky, S. L.; McMullen, A. K.; Niccolai, G. P.; Rothwell,
I. P.; Huffman, J . C. Organometallics 1985, 4, 902. (b) Bochmann, M.;
Lancaster, S. J . Organometallics 1993, 12, 633.
(26) Horton, A. D.; Orpen, A. G. Organometallics 1991, 10, 3910.
(27) The proximity of the two ZrMe resonances is surprising as in
[{(C5H5)2ZrMe}2(µ-Me)]+ the µ-Me resonates ca. 1 ppm upfield of the
terminal methyl: Bochmann, M.; Lancaster, S. J . Angew. Chem., Int.
Ed. Engl. 1994, 33, 1634.

2674 Organometallics, Vol. 15, No. 12, 1996
Communications
Sch em e 2
atactic propene oligomers (1H NMR: CH2dC(R)Me end
groups, Mn ) 5600), corresponding to a rate of 20 g/g
Zr‚h.
Sch em e 3
The new tridentate diamide ligand affords a relatively
robust framework for cationic group 4 alkyl complexes.
Although the ligand is subject to degradation by C-H
activation of a SiMe3 group adjacent to zirconium, the
reaction is much slower than that found by us for other
amide ligands.28 Similar to the situation for cationic
alkylmetallocenes, the cationic η2-benzyl adduct of the
tridentate ligand is a rather weak electrophile,23b whereas
the methyl cation coordinates the [MeB(C6F5)3]- anion,
NMe2Ph, or even N3ZrMe2. This work has shown that
cationic diamide systems can function as effective
ethene polymerization catalysts. The low propene
reactivity may reflect a general inertness of four-
coordinate diamide cations toward coordination and
insertion of alkenes larger than ethene. In other
studies, we have found that three-coordinate analogues
exhibit higher activity toward propene.28
amount of the cation reacts (giving unidentified prod-
ucts), reflecting slower initiation than propagation in
polymerization. In an autoclave experiment, benzyl
cation 4 (0.2 mmol, 7.1 bar ethene, 200 mL toluene, 25
°C, 1 L autoclave) afforded 11.0 g of polyethene in 10
min (rate 3600 g/g of Zr‚h; GPC analysis, Mw ) 230 000,
Mn ) 52 200). The deviation of the polydispersivity (Mw/
Mn ) 4.4) from the ideal uniform site value of 2 is partly
ascribed to the nonisothermal nature of the polymeri-
zation (exotherm of +55 °C within 90 s). A similar
reaction with propene (6.6 bar, 60 min) gave 0.45 g of
Su p p or tin g In for m a tion Ava ila ble: Text and a table
providing full details of the preparation, characterization, and
alkene reactivity of the compounds (11 pages). Ordering
information is given on any current masthead page.
OM960073K
(28) Horton, A. D.; de With, J . Chem. Commun., in press.